This invention is directed to an optical device such as an optical isolator or an optical modulator, and more particularly to an optical device comprising a pair or tapered birefringent plates, or an optical isolator or an optical modulator utilizing said tapered birefringent plates.
Optical fiber communication systems are now in practical use, and efforts are being made to advance research and development in this field. Because of this, requirements for optical devices with more versatile functions have also increased. For example, optical circulators are required for two-way communications. Since optical polarization cannot in general be preserved in fibers, it is preferable to develop optical circulators having characteristics that are not affected by incident light polarization.
An optical isolator is used as a functional component in light transmission systems to realize a one way transmission of light. In a typical example of the isolator as illustrated in FIG. 1, there is provided a 45.degree. Faraday rotation isolator 3 which always rotates in one direction by virtue of a permanent magnet. A polarizer 2 and an analyzer 2' are respectively placed before and after the Faraday rotation isolator 3, with the polarizer 2 and analyzer 2' being maintained at relative positions rotated 45.degree. with one another.
Light emitted from an optical fiber 1 is divided or separated into parallel beams 5 by a lens 7, and of the parallel beams 5, the polarizer 2 allows only polarized light in a particular direction to pass through it, and any other light is reflected and eliminated. Polarized light that has passed through the polarizer 2 emanates from the Faraday rotation isolator 3 with its plane of polarization rotated 45.degree.. Analyzer 2' is so arranged that polarized light with its plane of polarization rotated 45.degree. passes through the analyzer 2', focussed by a lens 8 and enters an optical fiber 4. On the other hand, of light 6 coming in the reverse direction from the optical fiber 4, only polarized light which is rotated by 45.degree. relative to the polarizer 2 may pass through the analyzer 2'. Polarized light that has passed through the analyzer 2' will have its plane of polarization rotated 45.degree. by the Faraday rotation isolator 3, and then emanates therefrom. Thus, polarized light 6 that is rotated 90.degree. relative to the polarizer 2 and that emanates from isolator 3 to be reflected by the polarizer 2 is eliminated. Because of this, light in the forward direction propagrates forwardly while light in the reverse direction is eliminated. However, the isolator just described is polarization dependent even with respect to light in the forward direction. In other words, specific polarized light only can pass through the isolator in the forward direction, and remaining light is not effectively utilized because it is eliminated.
The present invention also concerns a phase difference-light intensity converter. Functionally, such a converter is used as a device to convert the phase difference caused by a phase difference modulator (such as a Faraday rotation isolator bringing about rotation of the plane of polarization or an electrooptic device causing elliptical polarization) to transmission light intensity.
Another prior art device shown in FIG. 2 employs a polarizer 12 and analyzer 12'. In case of an electrooptic device, there is provided a phase difference modulator 13 which functions to give phase difference to two orthogonal components of the plane of polarization in response to an electrical input signal thereto, with polarizer 12 and analyzer 12' respectively placed before and after the modulator 13. Polarization axes of polarizer 12 and analyzer 12' are maintained at a predetermined angle .theta. with respect to each other depending on the usage of the device.
Light emitted from an optical fiber 11 is collimated into parallel beams by a first lens 16. Only a linear polarization component in a particular direction of parallel beam 15 is allowed to pass through the polarizer 12, and all other light is reflected and eliminated. Linear polarized light that passed through the polarizer 12 undergoes elliptical polarization by virtue of the converter 13 and is emitted. Of this light, only linear polarized light in the direction of the principal axis of the analyzer 12' passes through it, is focussed by a second lens 17 and then enters an optical fiber 14.
Therefore, where only the component having an angle of rotation of the plane of polarization (polarization plane rotation angle) .theta. (or .theta.+.pi./2) given by the converter 13 enters the optical fiber 14, the component having a polarization plane rotation angle .theta.+.pi./2 (or .theta.) is reflected by the analyzer 12' and does not enter the optical fiber 14. Thus, what is shown is a device converting the phase difference to light intensity which allows the component having the polarization plane rotation angle .theta. to pass through it.
However, the device just described is polarization dependent relative to incident light from the optical fiber 11. That is, of incident light, only the component having a specific polarization plane is allowed to pass through the polarizer 12, with all other light eliminated and not utilized at all.
Japanese Patent Publication (Unexamined) No. 149046 of 1978 published on Dec. 26, 1978 concerns an invention titled: Optical isolator. This isolator comprises first and second birefringent crystals and a Faraday rotator interposed therebetween. The Faraday rotator is characterized in that it rotates the polarization direction of each of polarized beams (2m.+-.1/2).times.90.degree. and (2n.+-.1/2).times.90.degree., where m and n are arbitrary integers, and that it is a non-reversible light rotating device.
Japanese Patent Publication (Unexamined) No. 79060 of 1979 published on June 23, 1979 is for an invention titled: Optical modulator. This modulator comprises a first crystal member which is capable of rotating the plane of polarization of light beam as much as 90.degree. when electric voltage is applied thereto, and it is interposed between second and third birefringent crystal members formed of a uniaxial crystal. The faces of the second and third crystal members are placed in the path of light and arranged parallel. The optic axis of each of the second and third crystal members is so inclined relative to said face that ordinary rays and extraordinary rays form a predetermined angle. A luminous flux of light entering the second crystal member is emitted therefrom as ordinary ray and extraordinary ray separated from each other with a predetermined distance. Ordinary rays and extraordinary rays then enter the first crystal member, with the direction of polarization of each ray being rotated 90.degree. or 0.degree. depending on whether electric voltage is applied to the first crystal member or not. Ordinary rays and extraordinary ray, whether rotated do not, enter the third crystal member and, due to the birefringency of the third crystal, these rays are combined at a predetermined position where an output terminal is provided or cancelled out.
In an article titled: Polarization-independent isolators for fiber optics, which was published in the Journal of Denshi Tsushin Gakkai (Electronic Communications Association), 1979/7 Vol. J. 62-CNo. 7, pp. 505-512, isolators schematically illustrated in FIGS. 3A, 3B and 3C are described. In these figures, reference numerals 21 and 22 denote optical fibers, 23 and 24 birefringent crystals, 25 a 45.degree. Faraday rotation isolator, 26 a 45.degree. rotator which functions as a compensation plate, o denotes an ordinary ray, and e an extraordinary ray.
Although practical isolators for fiber optics independent of polarization have been disclosed, a combination of lenses for forming focal points as shown in FIG. 3C are required for the operation of the disclosed polarization-independent isolators. This is because in combining parallel beams, the beam that is slightly shifted and the beam that is not shifted can hardly be distinguished. Because of the lens arrangement as shown, fibers 21 and 22 must be kept apart a considerable distance. This makes miniaturization of isolators difficult. In addition, lens aberration becomes large in the shown combination, bringing about increase of both insertion loss and cross-talk, for example, insertion loss in the order of 5 dB and cross-talk in the order of -20 dB.